So my 01 Mustang GT is basically all stock, only upgrade I did was add Flowmaster 40s for that classic muscle car sound but I want it to become a bit louder without spending a ton of money, and without deleting cats completely or getting an O/R pipe because of emissions. Yes, I know I could get mil eliminator and whatnot but I really don't want a raspy sounding car, I want to maintain that classic muscle car sound but just a little more volume in the sound. Also, I read that even buying a new Catted H-pipe (that was 2.5") would barely make any difference because stock H-pipe is good enough. I did some research and looked into high flow cats, and since I'm still running factory everything (other than mufflers), would it be beneficial to swap out the cats for some high flow ones? Would they add any performance gains?
"High flow" is a marketing gimmick. A good set of new OEM cats are "high flow". Its when they get older and clogged up is when they crap out.
Just get a catted UPR pipe and be done with it. They use Magnaflow cats which are extremely good cats and the ground clearance/fit is perfect as they are made in Florida by a shop called VRS.
Replacing the mid pipe and catback is not going to do a ton power wise but a little. There is really not much you can do for the 4.6L bolt on wise for power. Its when you get into the cams or power adders that you start jumping it up noticeably. Performance wise if you are not going to get into any of that you are much better served with suspension and driveline mods. IE, 4.10 gears, subframes, springs, shocks, swaybars, control arms etc...